Why is bryophyllum a weed?

Environmental tolerances, heart, muscle and nerve toxicity, land invasions and multiple reproduction means account for bryophyllum's weed status. Especially the species Brophyllum daigremontianum ("mother-of-thousands") and delagoense("mother-of-millions") exemplify the genus's tolerance of drought stress and poor soils. They produce plantlets that drop from host plant leaves to form colonies as well as seeds that last for years in the worst above- and below-ground conditions.